N-type or p-type semiconductor metal oxides have been considered for the fabrication of gas detecting elements. These are thick films of the semiconductor oxide that have been observed to undergo electrical resistance changes on exposure to combustible gases such as CO and H.sub.2. These changes in resistance (R) are related to the concentration (C) of the combustible gas in oxygen-containing atmospheres. An experimentally established R versus C relationship, under conditions that simulate the environment of the application, is used to obtain the concentration of the combustibles.
Stannic oxide (SnO.sub.2) has been found to be the most useful n-type semiconductor oxide for the fabrication of thick film combustible gas detecting elements. The response of these elements to reducible type of gases becomes measurable and useful only between 200.degree. and 300.degree. C. and when the conducting oxide is intimately mixed with small amounts of a noble metal catalyst. The gas sensitivity and reproducibility of the detecting elements also require good film mechanical stability and optimum porosity. The electrical resistance in the reference measurement gas should be within a convenient range in order to minimize or eliminate electronic noise problems.
In the current low temperature (200.degree.-300.degree. C.) operated toxic gas detecting elements, the above requirements are achieved by repeated grinding and heating of the starting powder materials. In addition to the semiconductor oxide (SnO.sub.2) and the noble metal catalyst, these materials normally include additions of SiO.sub.2, Al.sub.2 O.sub.3 and MgO.
U.S. Pat. No. 4,397,888 assigned to the present assignee and incorporated herein by reference, describes an improved stannic oxide thick film sensor for H.sub.2 and CO. That patent teaches the inclusion of specific catalysts and other additives in SnO.sub.2 and a unique method for the preparation of the stannic oxide gas detecting film.
It has been the practice to employ stannic oxide-based thick film sensors as detectors of toxic gases in the ambient air environment. For such pollution oriented applications, a small spiral or film resistance heating element is normally used to heat the film sensor to its operational temperature which is approximately 200.degree. C. or greater.
There is a present need to use thick film gas sensors for the in situ monitoring of the concentrations of residual combustible gases in operating gas boilers in order to effect a proper adjustment of the air-to-fuel gas ratio. Such adjustments are required to achieve a desired control of, and optimal efficiency in, the combustion process. In situ sensor operation presents additional constraints. For example, in an operating gas boiler, the combustible gas, thick film sensor is exposed to N.sub.2 /CO.sub.2 /H.sub.2 O vapor/O.sub.2 gas mixtures and temperatures as high as at least 500.degree. C. The concentration of CO.sub.2 and H.sub.2 O in such vapor combustion products is expected to be about 20 and 11 percent, respectively; the concentration of O.sub.2, between 5 to 0.1 percent. The balance will be nitrogen. Therefore, the gas film sensors must be more thermally stable than those currently used as toxic gas detectors. Moreover, the sensors must be reproducible and reversible in such a rigorous environment.
It is therefore an object of this invention to provide a thick film gas sensor suitable for a variety of in situ combustion gas monitoring applications.
It is another object of this invention to provide an antimony-doped stannic oxide sensor which is more electronically active and thermally stable than existing stannic oxide based sensors.
It is a further object of this invention to provide a combustible gas sensor having an extended temperature range of response between about 200.degree. C. and at least about 550.degree. C.
It is still another object of this invention to provide a unique method for the fabrication of thick film gas sensing elements, which method facilitates the convenient synthesizing of antimony-doped stannic oxide material and the manufacture of a gas detecting film.